The present invention relates to a method and device for the measurement of short distances by the analysis of the delay in the propagation of a wave.
It can be applied notably to anti-collision devices for vehicles, to distance-measuring instruments used for ships drawing alongside, or to any equipment that necessitates the detection, at a few meters, of the presence of a target for example. More generally, it can be applied to any instrument that carries out a measurement of the distance between itself and an obstacle that it is approaching, the range of the distances involved extending from zero to several tens of meters for example.
At present, instruments carrying out a distance measurement at less than some tens of meters are based either on a system of optical triangulation or on an analysis of the variation in the amplitude of the signal reflected by the obstacle. These methods are well known to those skilled in the art. However, measurement by optical triangulation is not compatible with unfavorable meteorological conditions such as rain or fog for example. This method therefore heavily penalizes applications for the measurement of short distances wherein any operation with uncertain results is unacceptable. The measurement by analysis of the variation in amplitude of the signal reflected by the obstacle relies on the attenuation of this signal relative to that emitted by the measuring instrument. When the obstacle is like a large-sized plane for example, the relative attenuation varies as the square of the distance to be measured while, in the case of a localized obstacle, the attenuation varies as the power of 4 of this distance. This method, based on electromagnetic emission, can work irrespectively of the meteorological environment However, it entails the assumption that the reflectivity of the obstacle does not vary when the measuring instrument approaches the obstacle. This is rarely the case in practical uses. Thus, for example, the vulnerability of this method can be seen in the case of an anti-collision device for vehicles where the reflectivity of these vehicles, for the majority of waves including electromagnetic waves, varies according to the angle at which they are seen, this being an angle that is notably variable when a trajectory is being changed. There also exist systems that measure the delay of acoustic propagation on the journey to and from the obstacle, but these systems rely on the knowledge of the velocity of sound: they are therefore dependent on the working altitude and are incompatible with spatial applications. Finally, they can easily be jammed, either unintentionally by the environment or deliberately by means of amplifiers with defined delays for example, and are therefore practically not used. The best known methods used to overcome the effects of the reflectivity of the target are the already-mentioned method of optical triangulation, or the measurement of the delay of propagation of a wave. In order to withstand meteorological conditions in particular, the measurement of the delay of an electromagnetic wave is presently the approach that is universally adopted when the distances to be measured exceed several tens of meters. This solution also makes it possible to overcome the constraints related to the speed of approach and therefore ensures a certain independence between distance and speed. The electromagnetic devices used in these methods furthermore perform very precise measurements of distance. However, the smaller the distance to be measured, the more complex and costly are the measuring devices. Indeed, these instruments all use a parameter known as distance resolution. This parameter determines a segmentation of the distance into adjacent compartments. A precise measurement of the distance consists then in comparing the energy level of the signal received from the target in two adjacent compartments to determine the real distance to the target by weighting the distances to the centers of these two compartments. For measurements of short distances, this method would necessitate a considerable raising of the frequency band of the emitted signal.
For example, to measure a distance of 1.5 meters, the distance resolution should be typically one meter and should in no case be more than 1.5 meters. The electromagnetic wave should then have a spectral width equal to 1.5 MHz and a pulse duration of 6.6 nanoseconds. This complicates the implementation of the measuring devices which become costly and unreliable.